Transcript Introduction to Skeletal Muscle

Skeletal Muscle
Gross muscle
Plasma membrane
Neuromuscular junction
Action potential
Muscle Connective Tissue
• provides structure & form to muscle
• allows force to be transmitted to
tendons/bones
• three layers of connective tissue-composed primarily of collagen fibers
– epimysium (outer layer)
– perimysium (groups fibers into bundles
(fascicles))
– endomysium (surrounds each fiber)
Muscle
Connective
Tissue
Skeletal Muscle
Endomysial
connective tissue
within skeletal muscle
Connective Tissue Functions
• provides “scaffolding” upon which fibers can
form
• holds fibers together
• perimysium provides conduit for
arterioles/venules and intramuscular nerves
• distributes strain/force over entire muscle
• endomysium conveys part of contractile force
to tendon
• fibers taper near tendon attachment; folding
of plasma membrane
Myon
Myonuclei of skeletal fiber
Sarcolemma
• surrounds each fiber and composed of:
– basement membrane (outer side)
– plasma membrane
• basement membrane contains:
– acetylcholinesterase
– collagen
• functions of basement membrane
– termination of synaptic transmission
– attachment of fiber to endomysium
– scaffolding for muscle fiber regeneration
Plasma Membrane
plasma membrane composed of lipid bilayer
– has fluid properties
– regulates fiber ion concentrations with
membrane protein pumps and channels
Plasma Membrane Proteins
• myonuclei and satellite cells
– bound to inter surface of plasma membrane
• peripheral proteins (plasma membrane receptors)
– associated with surface of bilayer
– e.g., adenylate cyclase, kinases, hormone receptors
– integrins
• class of connective proteins
• link basement membrane to plasma membrane and
cytoskeletal structures
• integral proteins function as “gatekeepers”
– embedded in phospholipid bilayer
– selectively let ions pass
Methods of transport
•
•
•
•
osmosis (i.e., water)
simple diffusion (e.g., O2, CO2)
facilitated diffusion (e.g., glucose, lactate)
active transport (e.g., Na+, K+)
• several thousand
`
amino acids arranged
in 1 or more subunits
• hundreds of sugar
residues linked
• controlled by voltageor receptor-regulated
gate
Transport Times
 movement of side chain on protein
10-10 s
 movement of Na+ through a pore
10-8 s
 fastest enzyme turnover
10-6 s
 activation of a channel (rate-limiting step)
10-4 s
 actin-myosin turnover
10-2 s
Membrane potential (mV)
+20
0
-20
-40
-60
-80
Time (ms)
Na+
K+
Na+
Na+
Na+
Na+
K+
Na+
Na+
Na+
Na+
channel
K+
K+
K+
intracellular
K+
K+
K+
Na+
Na+
Na+
Na+-K+
exchange pump
K+
Na+
Na+
K+
K+
Na+
ATPase
K+
ADP
Pi
K+
Na+
Na+
K+
channel
K+
K+
Na+
ATP
Na+
K+
K+
K+
K+
Na+
K+
Distribution of Na+-K+ pumps in skeletal muscle and muscle-nerve
bundles (N). Pumps are lit from exposure to a labeled antibody
Action Potential
 results from disturbance (e.g. electrical)
to membrane
 affects membrane permeability to Na+
and K+
 follows “all-or-nothing” principle
Phases of Action Potential
depolarization – influx of Na+
repolarization – efflux of K+
hyperpolarization – overshoot of K+ efflux
Action Potential
Motor Unit
Motoneuron
• inputs to motoneuron are both excitatory and
inhibitory
• continuous nerve from spinal cord to
neuromuscular junction
• are all myelinated
–
–
–
–
wrapped with myelin (Schwann cells)
nodes of Ranvier
AP conducted by saltatory conduction
greatly  conduction velocity
• extrafusal motor units innervated by 
motoneurons
Motor End Plate
Neuromuscular Junction
 AP at motor end plate (active zones) causes Ca2+
influx
 stimulates vesicles to migrate/fuse to membrane
and release acetylcholine (ACh)
 ACh diffuses across synapse and binds with
postsynaptic ACh receptors
 most ACh metabolized by cholinesterase
 postsynaptic ACh binding causes Na+ influx and
K+ efflux
 depolarization causes development of APs
 Curare blocks ACh receptors
 Anticholinesterase drugs (e.g., mustard
gas, sarin) prevent hydrolysis of ACh
 Botulism (bacterium) blocks release of
ACh